Global Witness was instrumental in establishing KP in 2003, following its exposure of the trade in "blood diamonds" – diamonds that originate from conflict areas. KP is a certification scheme that sets out requirements for controlling the trade in rough diamonds through national legislation. It has 50 participants (including the European Union, which represents its 27 member states) and members account for around 99.8% of the global production of rough diamonds.

However, Global Witness said on Monday it was withdrawing as an official observer of KP, arguing the scheme had become irrelevant. There was little political will by governments or industry to clean up practices, it said.

Over the past nine years, KP had failed to evolve or address "the clear links between diamonds, violence and tyranny", Global Witness said in a statement.

"Nearly nine years after the Kimberley Process was launched, the sad truth is that most consumers still cannot be sure where their diamonds come from, nor whether they are financing armed violence or abusive regimes," said Charmian Gooch, a founding director of Global Witness. "The scheme has failed three tests: it failed to deal with the trade in conflict diamonds from Ivory Coast, was unwilling to take serious action in the face of blatant breaches of the rules over a number of years by Venezuela, and has proved unwilling to stop diamonds fuelling corruption and violence in Zimbabwe. It has become an accomplice to diamond laundering – whereby dirty diamonds are mixed in with clean gems."

"Over the last decade, elections in Zimbabwe have been associated with the brutal intimidation of voters. Orchestrating this kind of violence costs a lot of money. As the country approaches another election there is a very high risk of Zanu PF hardliners employing these tactics once more and using Marange diamonds to foot the bill. The Kimberley Process's refusal to confront this reality is an outrage," said Gooch.

Anne Dunnebacke, senior campaigner at Global Witness, told the Guardian the Zimbabwe incident was the catalyst for the organisation's withdrawal, but "by no means was this the only reason".

"We've been pushing for reform within the process on a number of issues for many years," said Dunnebacke. "Over the course of the nine years some improvements have been made and we've seen instances where reforms have been adopted. But in recent years that has not been the case. There is next to no political will by governments or industry to reform and make it credible."

She said more pressure needs to be placed on the industry to take action. Just as international industry standards have been developed and imposed in the sourcing of minerals, and in the use of sweatshops, the same principles need to be applied to the diamond trade, said Dunnebacke.

She added that Global Witness would continue to work with NGOs and civil society to reform the diamond sector, but it will now be putting its resources into "something with more chance of working" .

Last month, the KP civil society coalition of NGOs, which includes Global Witness and groups from Liberia, Guinea, Brazil and Sierra Leone, boycotted the KP plenary session held in the Democratic Republic of the Congo to express their lack of confidence in the process. In 2010 the coalition formally walked out of the biannual KP meeting.